I've seen PayPal used in 2 ways :
1.) The customer fills out the Billing and Shipping address information at the website (store) they are buying from,
clicks a "PayPal" button, and is sent to PayPal to pay the bill.
2.) Customer clicks a "Pay now with PayPal" button and is sent directly to PayPal
(without filling out any Billing and Shipping address information at the store they were at).
After customer pays at PayPal, he is sent back to the store, and all his Billing and Shipping address information
gets filled in automatically for the store : the info was transferred from PayPal to the webstore.
What are 1 and 2 called?
Option 1 is very easy to implement, but option 2 would take a lot of programming I guess.